linux & memory maps - uet lahore · don’t confuse physical & virtual address spaces •...

Linux & Memory Maps

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Presented by : Abdul Qadeer

Presented in Open Source Series Workshop 2010

22-24 December, 2010 © ICOSST 2010

• Virtual address space

– No one understands that outside the processor!

• Physical address space

Don’t confuse physical & virtual address spaces

• Physical address space

– You need that to do memory operations and many

other I/O operations

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int x = 25;

int address_of_x = &x;

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The C / C++ address of operator (&). Does it

return a virtual address or a physical address?




• VAS > PAS– Probably a normal scenario

• VAS == PAS– E.g. 32 bit VAS and 4GiB installed DRAM

– Some DRAM might go un-used and hence wasted!

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– Some DRAM might go un-used and hence wasted!

• VAS < PAS– Not a common scenario

– Might be used to physically isolate DRAM used by programs

– Physical address extension; 32 bit VAS, 36bit PAS




Can look VAS and PAS in /proc/cpuinfo

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Memory mapped I/O

• Part of PAS assigned to a device

– Writing to this mem => writing to device

– Reading from this mem => reading from device

– Devices negotiate for their required MMI/O region

• Old, dumb devices (e.g. ISA devices)

• Newer, smarter devices (e.g. PCIe devices)

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• DMA concerns

• MMI/O usage needs special care

– No compiler optimization (e.g. by using –O0)

– No pipeline reordering (use Linux read / write barriers)

– Disable caching (e.g. using bits in TLB)



Memory mapped I/O

• MMI/O PAS not usable as normal DRAM

– MMI/O does not take away actual DRAM!

– But does take away a region of PAS

– The corresponding DRAM might go wasted

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• Relocate actual DRAM to elsewhere

– VAS should be big enough for relocation

– Chipset should allow relocation



Memory mapped I/O

Can look PAS allocation in /proc/iomem

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Picture taken from Gustavo Duarte blog accessible at: http://duartes.org/gustavo/blog/post/motherboard-chipsets-memory-map



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I/O Controller Hub




MTRR registers

• Memory region registration process

– MTTR registers used

– BIOS tells some regions initially

– OS tries to find out the optimal registration

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• Looking Linux log in dmesg

– Shows interesting system information



MTRR registers

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MTRR registers

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MTRR registers

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How an Intel processor starts from 0XFF FF FF F0

Reset vector

18Picture taken from Gustavo Duarte blog accessible at: http://duartes.org/gustavo/blog/post/how-computers-boot-up/

All grey stuff is

memory mapped



How an Intel processor starts from 0XFF FF FF F0

Reset vector

Jump to main BIOS

19Picture taken from Gustavo Duarte blog accessible at: http://duartes.org/gustavo/blog/post/how-computers-boot-up/

All grey stuff is

memory mapped

This is due to X86

legacy



VAS distribution among Kernel and a user space process

• 3G/1G split

– Theoretically 2^32 VAS available to a process

– Kernel is mapped for every process for

performance reasons

– Can change this split at kernel compile time to say

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– Can change this split at kernel compile time to say

2G/2G or by a patch to 4G/4G

• Half / Half split in 64 bit systems

– 128 TB for processes and Kernel



VAS distribution among Kernel and a user space process

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32bit System 64bit SystemPicture taken from http://blogs.technet.com/b/markrussinovich/archive/2008/11/17/3155406.aspx?PageIndex=6 and http://en.wikipedia.org/wiki/X86-64



Process address space

• Process can use almost any part of user-VAS

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Figure taken from Virtual Machines, versatile platforms for systems and processes, ISBN-10: 1558609105





• Application binary interface (ABI)

– Conventions

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– Conventions

• Binary file format

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– ELF, COFF

• Language run-time

– C, C++ etc.

• OS conventions

– Loader, linker







– Conventions

• Binary file format

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– ELF, COFF

• Language run-time

– C, C++ etc.

• OS conventions

– Loader, linker




• Not everything in VAS is mapped

– Address 0 is invalid

– Guard page between stack and

heap

– Similar guard pages for thread

Guard pages

– Similar guard pages for thread

stack

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• See process virtual address map

– cat /proc/1/maps or cat /proc/self/maps

Guard pages

Address Range Rights Offset Device Inode File Associated

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• Use Linux

– May be via live CDs

• Compare address spaces

Learn how your computer is using memory

• Compare address spaces

– 32 bit

– 64 bit

– Specially interesting if you have 4 GiB or more

installed memory

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Linux & Memory Maps

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linux & memory maps - uet lahore · don’t confuse physical & virtual address spaces •...

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